The present invention relates, in general, to filters for electronic circuits, and more particularly, to a novel filter that utilizes piezoelectric resonators.
In the past, piezoelectric resonators have been used to replace electromagnetic filter elements in electronic circuits. Some implementations of these piezoelectric resonators include surface acoustic wave resonators and thin film bulk wave resonators. In order to obtain good frequency rejection, thin film bulk wave resonators generally are formed as a stacked piezoelectric resonator, and are most often composed of a multi-layer structure that has two piezoelectric films alternately positioned between three metal electrodes.
Bandpass filters that utilize stacked piezoelectric resonators generally have a narrow passband or range of frequencies that pass through the resonator. The passband's maximum width, or bandwidth, often is less than two percent of the passband's center frequency. Many communication applications require filters with a passband that has a high center frequency and a bandwidth that is in excess of two percent of the center frequency.
Accordingly, it is desirable to have a bandpass filter that has a passband which is greater than approximately two percent of the passband's center frequency.